Polymer compositions, such as polyethylene terephthalate (PET) and polypropylene (PP) are well known packaging materials. For example, U.S. Pat. No. 4,340,721 describes a PET composition used to manufacture beverage bottles and other containers (hereinafter referred to as “bottles”) by various molding methods.
Bottles made from PET, such as for mineral water and carbonated beverages, are generally made by injection stretch-blow molding. This technique involves the injection molding of a “preform” which is subsequently blow molded into the final bottle shape. This may be carried out on separate injection mold and stretch-blow machines or on a single machine where the two steps are combined. Preforms usually consist of a threaded neck with a shortened bottle body shape 8 to 20 cm long with a material thickness between 3 mm and 6 mm. In order to blow the bottle, the preform is reheated by infrared lamps to a specific temperature above the glass transition point of the PET, such that it can be stretched and blown into a mold of the desired shape.
In general, PET resins have a poor ability to absorb infrared radiation. The preform heating and stretch blow moulding stage therefore becomes a rate-limiting factor in the overall bottle production process. Furthermore, the preform heating step also requires a significant amount of energy. To address this, many grades of commercial PET bottle resin incorporate additives to improve the heat-up rate (hereinafter referred to as “faster reheat”) of the preforms. The aim is to increase the rate of blowing, and thereby the overall productivity, as well as to reduce the energy required to reheat the preform.
In practise, the additives used to improve reheat in PET are finely dispersed inert black materials that strongly absorb radiant energy at the wavelengths emitted by the infrared lamps (generally between 500 and 2000 nm) used in stretch blow moulding machines. Examples of the materials used in PET are carbon black, as described in U.S. Pat. No. 4,408,004, graphite as described in U.S. Pat. Nos. 5,925,710 and 6,034,167, black iron oxides as described in U.S. Pat. No. 6,022,920, iron phosphide and iron silicide as described in U.S. patent application publication 2003/0018115 A1 and black spinel pigments as described in U.S. patent application publication 2002/0011694 and U.S. Pat. No. 6,503,586. The addition levels of these additives, in order to obtain the desired level of reheat improvement, is generally between 5 and 100 ppm.
Improved reheat in PET has also been shown by the use of antimony metal particles. These particles are usually deposited by a chemical reaction between the antimony polymerisation catalyst and a reducing agent (for example phosphorous acid) during the melt polymerisation stage, as described in U.S. Pat. Nos. 5,419,936 and 5,529,744.
Whilst the reheat improvement described above generally applies to PET, a further consideration, and a main embodiment of this invention, is the improvement of reheat in PP resins. PP is increasingly replacing PET for bottles for many beverage applications due to its lower material cost. U.S. Pat. No. 6,258,313 teaches that injection stretch blow molding of a PP preform is possible if the preform is heated simultaneously both from the outside and inside. Nevertheless, until recently it has been more difficult to produce satisfactory beverage bottles from PP than PET by this method. Firstly, polypropylene has a lower density and specific heat than PET and hence exhibits a significantly narrower processing window. Secondly, polypropylene suffers from the same limitations as PET in terms of its poor ability to absorb IR radiation. In general, polypropylene also has a greater opacity than PET, which detracts from its aesthetic appearance. The industry therefore continues to seek ways to improve the IR absorption properties of polypropylene such that it can be used to make beverage bottles on the same injection stretch blow-molding equipment as PET.
For PET and PP resin manufacturers who do not wish or are unable to use other black body absorbers, a convenient additive for improved reheat is carbon black. Carbon black offers the advantages of inertness, low cost, and ease of dispersion in the resin compared to other absorbing materials. Carbon black also exhibits a high degree of absorption at near-infrared wavelengths. It also has a high emissivity and hence a high proportion of the increase in temperature of the particles resulting from this absorption is transferred to the surrounding polymer. Thus very low levels of carbon black need to be added to the polymer in comparison to other black materials.
In using these additives, bottle manufacturers aim to maximise the improvement in reheat whilst minimising the impact on the colour and haze of the final bottle. By definition, the addition of a black material to the resin leads to darker bottles that are perceived to be less attractive than perfectly colourless ones. A particular disadvantage of carbon black is the dark hue and yellow-brown color tone imparted to the resin containing even very small amounts of carbon black. This problem becomes increasingly apparent as manufacturers aim for progressively faster reheat rates. Black materials that meet a desired combination of reheat and color performance continue to be sought.